Joining and machining of(ZrB2-SiC)and(Cf-SiC)based composites

R.V.Krishnarao,G.Madhusudan reddy,V.V.Bhanuprasad

Defence Metallurgical Research Laboratory,Kanchanbagh,Hyderabad,500058,India

Keywords:ZrB2-SiC Composite Sintering Machining Gas tungsten arc welding

ABSTRACT Filler materials of(ZrB2-SiC-B4C-YAG)composite were developed for gas tungsten arc welding(GTAW)of the ZrB2-SiC and Cf-SiC based composites to themselves and to each other.Reaction with filler material,porosity and cracks were not observed at weld interfaces of all the joints.Penetration of filler material in to voids and pores existing in the Cf-SiC composites was observed.Average shear strength of 25.7 MPa was achieved for joints of Cf-SiC composites.By incorporation of Cf-SiC(CVD)ground short fibre reinforcement the(ZrB2-SiC-B4C-YAG)composite was machinable with tungsten carbide tool.The joint and machined composites were resistance to oxidation and thermal shock when exposed to the oxy-propane flame at 2300oC for 300 s.The combination of(ZrB2-SiC-B4C-YAG)and Cf-SiC based composites can be used for making parts like thermal protection system or nozzles for high temperature applications.

1.Introduction

Zirconium diboride(ZrB2)is well known for its oxidation property since 1960s.It has very attractive combination of properties such as high melting point,high chemical stability,high hardness and strength,and high thermal and electrical conductivities suitable for thermal and chemical environments in hypersonic flight,rocket propulsion,and atmospheric re-entry[1-3].It is a promising material for structural parts in high temperature environments such as high speed air craft leading edges and nose cone.Due to its covalent bonding,and low self diffusion coefficient,ZrB2based composites can be sintered to high density by hot-pressing(HP)[4-6].Through HP process,simple geometries and moderate sizes can only be made.Fabricating complex shapes by diamond machining is an expensive and time taking process.Pressure less sintering(PS)offers a cheap and near-net shaping of complex parts by minimizing machining.Large size or complicated shape components of ceramics can be fabricated at low cost by joining.

Carbon fiber reinforced silicon carbide(Cf-SiC)composites are well known for high strength and modulus,moderate fracture toughness,and high resistance to oxidation at high temperatures.They are considered promising materials for hypersonic aircraft applications particularly for heat shields and advanced propulsion structural components[7].Uniform and complete densification is difficult to achieve in large and complex shaped components.Joining is inevitable to develop Cf-SiC components for high temperature applications.There exist several techniques to join monolithic ceramics or ceramic matrix composites:brazing[8],diffusion welding[9],reaction joining process[10,11],precursor in filtration and pyrolysis[12],reactive melt in filtration[13]and chemical vapor deposition online joining process[14,15].But the joining strengths are still not enough for engineering application.Mechanical joining with screws,nails,bolts,and adhesives(glues and epoxies)is not suitable for high temperature environments[16].Welding and brazing of Cf-SiC composites is not easy because commonly used filler materials have little or no wetting.Brazing with suitable filler normally requires special surface treatment and equipment[17,18].Different layers of refractory borides(TiB2and ZrB2),carbides (SiC,B4C,and WC),or their mixtures(TiB2+SiC+B4C),were used for solid state diffusion bonding[19,20].Long exposures to high temperatures(about 2000 K)under relatively high loads are used to obtain a pore-free layer and strong joint.But long heat treatment at high temperature deteriorates the properties of parent composites.

Recently Cf-SiC composites were joined by spark plasma sintering(SPS)using(SiC+5 wt%B4C)powder mixture[21],Ti foil and calcia-alumina glass ceramic[22].All these techniques require rapid heating by SPS under a pressure of 60 MPa to a temperature of 1480-1900oC for 3-10min.In spite of heating to such a high temperature the shear strength oft he resulting joint was 17.3±7.8 MPa only.Ti3SiC2max phase produced by self propagating high temperature synthesis(SHS)was also employed for diffusion brazing in high vacuum hot pressing at 1600oC under 25 MPa pressure[23].

Different braze alloys are used tojoin ZrB2-SiC based composites to themselves and to commercially pure Ti[24,25].Chemical interaction with braze alloy and interfacial cracking due to residual stresses were observed.Due to melting of these solders the joints cannot be useful at temperature above 1000oC.Parts up to 3 mm thick of ZrB2-20 vol%SiC and ZrB2-SiC-B4C composites were joined by gas tungsten arc welding(GTAW)or plasma arc welding.Porosity at weld interface is unavoidable[26,27].

In an earlier work[28]a filler with high vol.%of B4C and YAG(ZrB2-25 vol.%SiC-25 vol.%B4C-16vol.%YAG)was used to join hot pressed(ZrB2-20vol.%SiC),and pressure less sintered(ZrB2-SiCB4C-YAG)composites to themselves.In this work a modified filler material of(ZrB2-SiC-B4C-YAG)with low quantities of(Y2O3,Al2O3,and B4C)[29]has been used to join Cf-SiC composites to themselves and to(ZrB2-SiC)based composites.The joint facing ZrB2-SiC composite was exposed to the oxy-propane flame(2300oC)in 30 s interval to examine the resistance of the joint to thermal cycling,ablation and oxidation.

Some degree of machining is always required to make complex and precision components.Even with diamond tools,laser machining and ultrasonic machining the machining cost usually accounts for 70-90%of the total cost[30].Electrical discharge machining(EDM)requires a material resistance and can only machine components of small size[31].Traditional mechanical machining is of both cost-effective and time-efficient.Mechanical machining of strong and hard ZrB2-SiC is very difficult.By introducing mica,h-BN,graphite,pores,rare-earth phosphates and Ti3SiC2the machinability of ceramic materials can be improved[32].The fabrication of a machinable ZrB2-SiC-BN composite by hot pressing at 1800oC and 23 MPa was reported[33].Though machinability increased the fracture toughness and hardness of the composite decreased due to the formation of large agglomerates or platelets of BN.Dense ZrB2-SiC-BN composites with fine grain size and homogeneous microstructure were fabricated via reactive spark plasma sintering of a mixture of ZrH2,Si3N4and B4C powders at 1900oC in vacuum[34].

A ceramic throat insert for a nozzle was machined by EDM of a composite prepared by hot pressing a mixture of 46 vol%ZrB2+8 vol%Si3N4and 46 vol%C chopped fibers at 2100 K and 30 MPa[35].Fiber degradation was effectively inhibited in ZrB2-SiC-Cf composites containing 20-50 vol%carbon short fibers using nanosized ZrB2powders and hot pressing at low sintering temperature(1450oC)[36].In the present work a machinable ZrB2-SiC based composite was pressure less sintered at relatively low temperature of 1580-1650oC.Scrap pieces of Cf-SiC composites were ground and the(-200,+325)mesh size powder obtained after sieving was used to reinforce the ZrB2-SiC based composite.The resultant composite was subjected to drilling and exposure to the oxy-propane flame at 2300oC.

2.Experimental

2.1.Materials

The ZrB2powder was synthesized via B4C reduction of ZrO2reaction(1).When B4C and ZrO2react in the stoichiometric wt.ratio of ZrO2/B4C≈3.0,the formation of ZrB2with impurities like ZrO2,B4C,and C occur due to the loss of boron as B2O3.To compensate the loss of boron as B2O3and to obtain a single phase ZrB2without impurities,excess of B4C was taken in a wt.ratio of ZrO2/B4C=2.5[37].ZrO2powder was supplied by Nuclear Fuel Complex,Hyderabad,India.B4C powder was purchased from China Abrasives,Zingzhou,China.Fine SiC powder with particle size(d50～0.8μm)was received from H.C.Starck,Germany.Super fine size(d50～0.7μm)Al2O3from Alcan and sub micron sized Y2O3were used.Cf-SiC composites processed by chemical vapor deposition(CVD)and reaction bonded silicon carbide(RBSC)were manufactured by CSIR,National aerospace laboratories,Bengaluru and DRDO,Advanced systems laboratory,Hyderabad respectively.The waste or scrape pieces of Cf-SiC(CVD)retained after fabricating test samples for flexural strength and oxidation evaluation are ground in a mortar with pestle.After sieving the groundpowder,the(-200 and+325#)fraction was used for reinforcing(ZrB2-SiC-B4C)composites.

2.2.Pressure less sintering of ZrB2-SiC based composites and filler welding rods

Y2O3and Al2O3were added as sintering additives for PS of(ZrB2-SiC-B4C)composite.The typical composition used was(ZrB2:SiC:B4C:Y2O3:Al2O3=65:20:8:3:4)vol.%.The mixing of dry powders was done for 24h with alumina balls in a polythene bottle.Compacts of 60 and 30 mm diameter were made by uni-axial compaction with a load of 9 Tons and 3 Tons respectively.PS of ZrB2-SiC based composites at 1650oC in argon atmosphere for 1.0 h was carried out in a carbon furnace(ASTRO,USA,Model 1000-3060-FP20)[38].The bulk density of the PS composite is 4.52g?cm-3.The Vickers microhardness with 200 g load was about 12.53 GPa and its flextural strength was 213 MPa.Further,a 20 vol%of Cf-SiC(CVD)powder of(-200 and+325#)size was added to above(ZrB2:SiC:B4C:Y2O3:Al2O3=65:20:8:3:4)vol.%composite and pressure less sintered.

For GTAW of ZrB2-SiC based composites a filler with high vol.%of B4C and YAG(ZrB2-25 vol.%SiC-25 vol.%B4C-16 vol.%YAG)was used[28].The vol.%of Y2O3,Al2O3and B4C in the filler was decreased to(ZrB2:SiC:B4C:Y2O3:Al2O3=65:20:8:3:4)to allow rapid deposition of high volume of molten filler to flow into the pores existing in the Cf-SiC based ceramics.Dry powders of ZrB2,SiC,B4C,Y2O3and Al2O3were mixed for a minimum of 8h.PVA binder in water solution is used to make a thick paste.The paste obtained is extruded through a nozzle or medical syringe to obtain a wire/rod of about 3mm diameter.Alternately green compacts of filler material were made by uniaxial pressing in a steel die.The extrusions or compacts after drying in a oven at 110oC for 1h were pressure less sintered at 1650oC.Using diamond cut off wheel or electro discharge wire cut machine,bar samples of size 3×3 and 55-75 mm long were cut from sintered compact.The sintered extrusions or cut bars were used for GTAW of Cf-SiC composites to themselves and to ZrB2-SiC based composites.

2.3.Joining of(ZrB2-SiC)and(Cf-SiC)based composites

The Cf-SiC composite and ZrB2-SiC based composite of size 4 mm×5 mm×50 mm long were used for joining by GTAW.The coupons to be joined are kept on steel platform.Keeping a Cf-SiC composite piece and another Cf-SiC composite piece or ZrB2-SiC based composite piece at a distance or gap around 1 mm.A square butt weld configuration was employed in the present study.Welding parameter employed are:90-120 A current,and speed of 3mm?min-1.After welding the argon flow was continued till the joint was cooled to a temperature below 800oC.The joining was repeated on the opposite side.A Cf-SiC(CVD)composite piece of 20 mm square was joined to a cylindrical sample of(ZrB2-SiC-B4CYAG)composite of 27mm diameter and 7.5 mm height.Finally,bar samples of(ZrB2-SiC-B4C-YAG)composite reinforced with Cf-SiC(CVD)powder were also welded to themselves.

2.4.Characterization

The joints were cut perpendicular to the weld direction with diamond cut off wheel or wire cut electro discharge machine(EDM).The cut pieces were mounted in epoxy to polish to mirror finish with fine diamond(0.25μm)abrasive.Microstructure was analyzed with a scanning electron microscope(SEM of FEI Quanta 400,Netherlands)equipped with energy dispersive spectroscope(EDS).Shear test specimens were prepared as per ASTM A 264 standard.Stainless steel fixture and Walte-Bai Ag,HTV-1200,universal testing machine(UTM)and 0.1mm?min-1cross head speed were used to determine the shear strength of the weld.The flexural strength with specimens of size 4 mm×5 mm×50 mm,span of 40 mm and cross-head speed of 0.5 mm?min-1was determined on Instron(UTM)of model No:8801.With a load of 200g for a dwell time of 15s the Vickers micro-hardness was measured with DM H-2,Matsuzawa Seiki,Japan.

The jointof ZrB2-SiC and Cf-SiC(CVD)composite was exposed to the oxy-propane flame of 2300oC on ZrB2-SiC side for 30 s for each exposure.The temperature of the flame and the sample immediately after withdrawing the flame was measured.The precision optical pyrometer was supplied by Pyrometer Instrument Co.,Inc.,USA.The sample was exposed for 10 times.After every exposure the weight of the sample was measured with a digital balance with resolution±0.1×10-4g.With a tungsten carbide tool a 3mm dia.hole was drilled in 30 mm dia.and 10 mm height sample of ZrB2-SiC based composite reinforced with Cf-SiC(CVD)composite powder.The sample with 3 mm hole was also exposed for ten times to the oxy-propane flame of 2300oC for 30 s each exposure.

3.Results and discussion

3.1.Joining of Cf-SiC composites

Joining of Cf-SiC composites by fusion welding is not possible because carbon and SiC sublimate/dissociate upon heating to temperatures above 200oC.ZrB2melts when an arc is struck with tungsten electrode.The molten liquid will flow between the solid surfaces of the composite.The solid surfaces are bonded together when the molten pool is cooled.The shrinkage during the solidification of the molten composite causes the formation of porosity at solid-liquid boundary between parent material and fusion zone.Formation of gases like SiO,CO,and B2O3due to the oxidation of the parent ZrB2-SiC composite can also induce porosity in the fusion zone.Though Cf-SiC composites do not under gomelting the molten pool formed from ZrB2-SiC based composite is expected to facilitate joining.But sublimation and dissociation of Cf-SiC at high arc temperatures results in failure of joining due to evolution of SiO and CO.Striking an arc between tungsten electrode and ZrB2-SiC weld coupon alone without effecting Cf-SiC weld coupon is not possible.

Without disturbing weld coupons,arc can be struck between suitable filler rod/wire and tungsten electrode.Filler melts and form a liquid pool in the gap between the composite surfaces to be joined.It is like pouring of molten metal into a suitable mold.The welding speed can be controlled to adjust the flow of filler into weld gap to avoid cracks and pores that could form due to shrinkage of molten filler.The wet ability of commonly used filler materials with carbon materials is little or nil.Even with wet able filler material,brazing requirespecial surface treatment.Due to low melting point of the generally used brazing alloys the joints cannot be useful at temperature above 1000oC.Chemical interaction,hair line cracks and interfacial cracking due to residual stresses were observed.For diffusion bonding,high temperatures,pressures,special fixtures and equipments are also required.

Cf-SiC composites irrespective of their processing route(CVD or RBSC)always contain porosity.To obtain a strong weld the filler should penetrate into pores in the Cf-SiC composite.This requires rapid deposition of filler material into weld gap.The filler used for joining of ZrB2-SiC based composites contains high volumes of B4C,Y2O3and Al2O3.BSE image analysis of filler after arc welding revealed the presence of different phases(Fig.1(a)).Through EDS analysis(Fig.1(b))different phases in filler of the joint were identified as bright phase-ZrB2,light grey phase-SiC,dark grey phaseytrria alumina silicate(YAG)and dark phase-complex yttria alumina-silicate containing SiC/ZrC.Among these phases B4C is highly prone to oxidation.Oxidation of B4C occurs by the formation of liquid boron oxide(B2O3)on the surface and its loss due to direct evaporation at temperatures above 1770 K[39].Due to the oxidation of B4C and its evaporation as B2O3during arc melting,it is not detected in the filler of the joint(Fig.1(b)).In the modified filler the volume%of YAG phase and B4C was decreased by lowering the initial volume%of B4C,Y2O3and Al2O3.The resultant filler composition,facilitate the rapid deposition of filler and its flow ability into Cf-SiC composite.

The appearance of joint of Cf-SiC(RBSC)composite is shown in Fig.2(a).The weld looks like a metallic joint.The weld inter face is continuous and free from reaction,cracks and porosity(Fig.2(b)).Since the welding is performed on both sides,the filler flown into butt weld gap from both directions and joined at the centre within the weld gap.Apart from the flow ability,the filler exhibited very good wet ability with Cf-SiC(RBSC)composite.The filler has penetrated into pores and gaps on both sides of the Cf-SiC(RBSC)composite(Fig.2(c)).The filler in the joint appeared like a single piece as if Cf-SiC(RBSC)composite is riveted from both sides with ZrB2-SiC based filler composite(Fig.2(b)).The filler could penetrate laterally into Cf-SiC(RBSC)composite.EDS revealed the interface between Cf-SiC(RBSC)composite and filler(Fig.2(d))is clean and free from reaction products.Penetration of filler into Cf-SiC(RBSC)composite far away from interface is observed in Fig.2(e).The EDS analysis of the selected window areas in Fig.2 revealed the presence of filler constituents only(Fig.3(a)and(b)).

Similar observations were recorded for joint of Cf-SiC(CVD)composite in Fig.4.The appearance of the joint is similar to that of joint of Cf-SiC(RBSC)composite.In this joint also the filler composite flown into weld gap from both sides and joined at the centre.During cutting and polishing a small piece at centre of the weld gap was broken and dislodged(Fig.4(b)).Penetration of filler into the Cf-SiC(CVD)composite can be noticed in Fig.4(c).The inter face between the filler and Cf-SiC(CVD)composite free from reaction products,agglomeration,porosity and cracks(Fig.4(d)).The typical dendritic structure of solidified filler is identified.The deep penetration of filler into Cf-SiC(CVD)composite(Fig.4(e))can enhance its performance at high temperature.The EDS analysis of the area marked by window in Fig.4(e)identified the constituents of filler material(Fig.3(c)).All the compounds/phases(ZrB2,SiC,and YAG)are compatible with carbon composites for oxidation protection[40,41].

3.2.Joining of Cf-SiC composites to(ZrB2-SiC-B4C-YAG)composite

The appearance of joint of Cf-SiC(CVD)composite to(ZrB2-SiCB4C-YAG)composite is shown in Fig.5(a).The two dissimilar ultra high temperature ceramics with difference in density,porosity and melting or sublimation temperatures could be joined without any difficulty.Without preheating or controlled cooling after welding,the joining was successful.The solidified dendritic structure and penetration of filler into Cf-SiC(CVD)composite can be observed(Fig.5(b)).The deep penetration of filler in this joint compared to Cf-SiC(CVD)joint in Fig.4 is due to the restriction of penetration in to dense(ZrB2-SiC-B4C-YAG)composite.In spite of two dissimilar materials the weld looks like a metal joined with filler.The weld inter face is free from cracks,agglomeration and porosity(Fig.5(b)).

Similar results were observed for joint of Cf-SiC(RBSC)composite to(ZrB2-SiC-B4C-YAG)composite(Fig.6).The flexibility or ease of joining to fabricate complicated shapes has been examined by making symbols like Pie and E.The typical Pie shape joint is shown in Fig.7(a).The two dissimilar materials Cf-SiC(RBSC)composite and(ZrB2-SiC-B4C-YAG)composite could be welded without any cracks.The inter faces between filler and parent composites Cf-SiC(RBSC)and(ZrB2-SiC-B4C-YAG)are free from voids,cracks,precipitation and reaction products(Fig.6(b)).The compounds in Cf-SiC based composites and(ZrB2-SiC-B4C-YAG)composite are compatible with each other.The filler and ZrB2-SiC based composites are similar with variation in volume%of individual compounds.The success of the joint is due to wet ability and flow ability of filler on Cf-SiC based composites and(ZrB2-SiC-B4CYAG)composite.Deep penetration of filler into the Cf-SiC(RBSC)composite is due to restriction of penetration into dense(ZrB2-SiCB4C-YAG)composite.

3.3.Shear testing of the joints

Shear test specimens were prepared according to the standard of ASTM A 264.The shear strength of the weld was determined with a stainless steel fixture and UTM at a cross head speed of 0.1 mmmin-1.During the test joints failed in transverse direction in the parent Cf-SiC composites(Fig.7).This could be due to the strong bonding between filler material and composites.The weld interfaces are also continuous and free from reaction,porosity and thermal stress during re-entry heating conditions[43].The pressure less sintered(ZrB2-SiC-B4C-YAG)composite joined to Cf-SiC composites possesses a strength above 80 MPa.Since the pressure less sintered(ZrB2-SiC-B4C-YAG)composite possesses oxidation resistance and thermal shock resistance,it is expected to protect Cf-SiC composite from high temperature oxidation and ablation.

3.4.High temperature flame exposure of joint of(ZrB2-SiC)and Cf-SiC composite

The high temperature applications of Cf-SiC composites for heat shields and advanced propulsion structural components involve exposure to oxidizing atmospheres for extended time periods.The possibility of using the pressure less sintered(ZrB2-SiC-B4C-YAG)composite for thermal protection of Cf-SiC(CVD)composite structure is tested.The 27mm diameter and 7.5 mm thick(ZrB2-SiC-B4C-YAG)based composite is welded to Cf-SiC(CVD)composite and exposed to oxy-propane flame of 2300oC for 30 s for each exposure on(ZrB2-SiC-B4C-YAG)(Fig.8).The samples were exposed for 30 s and allowed to cool naturally to measure the weight.The weld sample was exposed for 10 times for total exposure time of 300s to oxy-propane flame.The appearance of as welded joint of Cf-SiC(CVD)-(ZrB2-SiC-B4C-YAG)is shown in Fig.8(a).The cracks.Penetration of filler composites in to voids and pores in the Cf-SiC composites resulted in failure in parent Cf-SiC composites(Fig.7).The shear strength recorded for different joints was:24.46 MPa for Cf-SiC(RBSC),18.72 MPa for Cf-SiC(CVD),34.97 MPa for Cf-SiC(CVD)-(ZrB2-SiC-B4C-YAG),and 24.79 MPa for Cf-SiC(RBSC)-(ZrB2-SiC-B4C-YAG).The average shear strength of different joints is 25.7MPa.

High mechanical performance is not required for thermal protection applications.The material should possess high oxidation resistance and thermal shock resistance[42].The high temperature ceramic wing leading edge is expected to undergo 80 MPa peak appearance of joint during and after exposure to oxy-propane flame for 300s is shown in Fig.8(b)and(c)respectively.The Cf-SiC(CVD)composite and the joint are not affected by exposure to oxy-propane flame for 300 s.After flame exposure the joint appeared very clean and neat due to the removal of carbon and other vapors deposited during welding.

A weight gain of 1.052%was recorded for the joint of(ZrB2-SiCB4C-YAG)-Cf-SiC(CVD)after exposure for 300s.Its equivalent weight gain of 2.69 mg?cm-2per 300 s is in agreement with the value obtained for similar composite exposed to oxyacetylene flame[38].Oxidation of(ZrB2-SiC-B4C-YAG)composite is the main reason for the weight gain.After 10 times exposure to the flame the joint is dimensionally stable and resistant to thermal shock.

When(ZrB2-SiC-B4C-YAG)composite is oxidized at high temperature during flame exposure the formation of yttria stabilized zirconia(YSZ)in a complex YAG glass occur on the oxidized composite surface.In the absent of YAG in the ZrB2composite the nonstabilized ZrO2alone cannot adhere to ZrB2at high temperatures because it under go cracking due to the difference in coefficient of thermal expansion between oxide scale and un-oxidized ZrB2matrix.Theweak bonding causes the spall of the oxide scale[44].In(ZrB2-SiC-B4C-YAG)composite the YSZ precipitates in complex YAG glass which adheres to parent composite and protect it fromfurther oxidation by preventing direct exposure to air.

The stability of the weld upon exposure to oxy-propane flame for 300 s is shown in cross section of the joint in Fig.9.The triple junction of filler and parent composites in Fig.9(a)shows the resistance of the weld to thermal cycling and oxidation at high temperature.The weld interface between filler and Cf-SiC(CVD)composite is un affected.Oxidation or cracking is not observed.EDS analysis of the selected area marked with window revealed that the filler away from interface(Fig.9(b))was also un affected.Similarly the interface between the filler and(ZrB2-SiC-B4C-YAG)composite remained as it was welded(Fig.9(c)).Since the filler and the composite are similar in composition and the melting point of filler is about 3000oC,no reaction or cracking due to thermal cycling is expected.

Thus the combination of Cf-SiC composites and(ZrB2-SiC-B4CYAG)composites by GTAW joining with(ZrB2-SiC-B4C-YAG) filler is shown to be useful for thermal protection applications at ultra high temperatures.The filler material and the weld joint could with stand from oxidation and cracking at high temperature during the exposure to oxy-propane flame.Further the thickness of(ZrB2-SiCB4C-YAG)composite can be increased to maintain the Cf-SiC composite at safe operating temperature.Main advantages of the present joining method are:It does not require very large size high temperature furnace to heat the entire joining components.No need to use special fixtures to hold or to apply pressure/load to the joining components.The time of joining is very less compared other joining processes.The filler material can with stand to the exposure of high temperature.

3.5.(ZrB2-SiC-B4C-YAG)composite reinforced with-Cf-SiC(CVD)composite powder

While making test samples of Cf-SiC(CVD)for flexural strength and oxidation evaluation small waste or scrape pieces are retained.They are ground in a mortar with pestle.After sieving the ground powder,the(-200 and+325#)fraction was used for reinforcing(ZrB2-SiC-B4C)composites.The morphology of the typical ground powder is shown in Fig.10(a).The ground pieces of short fibers or Cf bundles with the coating of CVD SiC appeared like whiskers.A 20 vol%of Cf-SiC(CVD)powder of(-200 and+325#)size was added to the(ZrB2:SiC:B4C:Y2O3:Al2O3=65:20:8:3:4)vol.%composite.After pressure less sintering at about 1650oC in argon atm.for 1.0 h the Cf-SiC(CVD)powder retained its morphology Fig.10(b).The bulk density of the composite 4.2g?cm-3is slightly lower than that of(ZrB2-SiC-B4C)composite(4.52 g?cm-3).The Vickers micro hardness with 200 g load was about 17.2 GPa.The hardness value is higher than that of(ZrB2-SiC-B4C)composite(12.5GPa).This could be due to increase in the total content of SiC.Variation in the value of Vickers hardness was observed due to the presence of soft phases(Cf and YAG)and hard phases(ZrB2and SiC).Large number of readings was taken and the average value was calculated.The flextural strength of the composite decreased to 120 MPa from 213MPa for(ZrB2-SiC-B4C)composite.The typical large grain size due to the liquid phase sintering and uniform distribution of Cf-SiC(CVD)powder and other phases is observed in Fig.10(b).

Using a WC tool bit a 3mm dia.hole was drilled into a 30 mm dia.and 10mm height PS compact(Fig.10(c)).In spite of the high hardness of the composite,drilling was possible due to the presence of Cf-SiC(CVD).Slight chipping away of the material during initial drilling on front side of the hole was observed(Fig.11(a)).The inner surface of the cylindrical hole and the rear or exit side of the hole are very neat and uniform(Fig.11(b)).Even after drilling the Cf-SiC(CVD)powder particles embedded in the(ZrB2-SiC-B4C)composite can be seen with naked eye in Fig.11(b).Thus,with the reinforcement of Cf-SiC(CVD)powder the drilling of the pressure less sintered(ZrB2-SiC-B4C)composite was possible with WC tool.Utilization of the scrap of very expensive Cf-SiC(CVD)composites was also realized.

The modified filler used for joining the Cf-SiC composites was also used for joining of the(ZrB2-SiC-B4C-YAG)composite reinforced with-Cf-SiC(CVD)composite powder.The joint of(ZrB2-SiC-B4C-YAG)-Cf-SiC(CVD)composite is shown in Fig.11(c).The joint was very neat and exhibited shear strength of 60 MPa.The shear strength of this joint is higher than that of Cf-SiC(CVD)joint and lower than that of(ZrB2-SiC-B4C-YAG)joint[28].

3.6.High temperature flame exposure of(ZrB2-SiC-B4C-YAG)composite reinforced with-Cf-SiC(CVD)composite powder

There is a great demand for advanced materials with capability of high temperatures exceeding 2000 K.High temperature applications include heat shields and advanced propulsion structural components involve exposure to oxidizing atmospheres.The high temperature capability of(ZrB2-SiC-B4C-YAG)composite reinforced with-Cf-SiC(CVD)composite powder was examined by exposing to the oxy-propane flame of 2300oC.The pressure less sintered compact of 30 mm dia.and 10 mm height after drilling a 3 mm dia.hole at the centre was exposed to the oxy-propane flame of 2300oC for 30 s for each exposure.The temperature of the flame and the compact immediately after with drawl of the flame was measured with a precision optical pyrometer.The compact was exposed for 0.5 min and allowed to cool naturally to measure the weight.The compact was exposed for 10 times for total exposure time of 300s.The appearance of the compact after exposure to the oxy-propane flame for 300 s is shown in Fig.12(a).After exposure for 300 s the composite retained its shape and dimensions without any cracking(Fig.12(b)).The weight gain of compact after exposure to the oxypropane flame is very low(0.27%).Its equivalent weight gain of 2.8 mg?cm-2per 300s is much lower than the value obtained for similar(ZrB2-SiC-B4C-YAG)composite without reinforcement of Cf-SiC(CVD)composite powder exposed to oxyacetylene flame[38].

Monolithic ZrB2oxidizes to form ZrO2and B2O3liquid at temperatures as low as 450oC.With addition of SiC the silicon and boron in the composite oxidizes at elevated temperatures and the resultant oxides react to form a protective borosilicate glass.

At temperatures above 1400oC volatilization of B2O3exceeds the production of B2O3.The weight gain due to the formation of ZrO2is greater than the weight loss due to the evaporation of B2O3.ZrO2dissolves in borosilicate glass and form BSZ glass channels between the oxide layer and un oxidized substrate.Oxidation of the bulk composite occurs with the escape of gas species of CO,and SiO through BSZ glass.Progressive evaporation of B2O3causes the change in microstructure of the oxide scale.

The viscosity of the glassy phase increases and the solubility of ZrO2in it decrease to cause ZrO2precipitation from BSZ glass.Formation of a complex YAG glass containing Zr was observed(Fig.12(c)).Upon high temperature flame exposure of(ZrB2-SiCB4C-YAG)composite,formation of yttria stabilized zirconia(YSZ)in YAG glass occur on the oxidized surface.In monolithic ZrB2,ZrO2cannot adhere to ZrB2as it under go cracking due to the difference in coefficient of thermal expansion between oxide scale and unoxidized ZrB2matrix.This causes weak bonding and results in spall of the oxide scale.In(ZrB2-SiC-B4C-YAG)composite the YSZ precipitates from BSZ glass and remain embedded in complex YAG layer.YAG that adheres to parent composite protects it from further oxidation by preventing direct exposure to air(Fig.12(d)).The EDS analysis of the oxidized surface of the flame exposed compact revealed the formation of complex YAG glass containing Zr(Fig.13(a))and precipitation of ZrO2embedded in YAG(Fig.13(b)).By employing joining and machining,parts like thermal protection system or nozzles for high temperature applications can be made at relatively low cost from combination of(ZrB2-SiC-B4C-YAG)and Cf-SiC based composites.

4.Conclusions

Using a filler material of(ZrB2-SiC-B4C-Y2O3-Al2O3)composite,gas tungsten arc welding of Cf-SiC composites to themselves and to ZrB2-SiC based composites was performed.The joints and interfaces were very clean and free from porosity and cracks.Filler material has penetrated in to voids and pores existing in the Cf-SiC composites.The Cf-SiC joints achieved an average shear strength of 25.7MPa.The(ZrB2-SiC-B4C-YAG)composite was machinable with tungsten carbide tool by incorporation of Cf-SiC(CVD)ground powder reinforcement.The filler material,joint and machined(ZrB2-SiC-B4C-YAG)composite reinforced with Cf-SiC(CVD)ground powder could with stand the oxidation and thermal cycling during exposure to the oxy-propane flame at 2300oC for 300 s.The(ZrB2-SiC-B4C-YAG)composite reinforced with Cf-SiC(CVD)ground powder can be used for thermal protection system or nozzles for high temperature applications.

Acknowledgments

Authors thankfully acknowledge the financial support from the Defence Research and Development Organisation,Ministry of Defence,Govt.of India,New Delhi in order to carry out the present study under project DMR-295.They are grateful to the Director,DMRL,Hyderabad,for his constant encouragement.The Authors acknowledge the support of XRD,SEM groups of DMRL.